The present invention relates to a cemented carbide body useful in applications where extreme cyclic loads and friction forces occur, creating high temperatures and rapid thermomechanical fatigue.
Continuous excavation methods for cutting of soft rock, minerals and roads, such as roadheading, continuous mining, road and concrete planing and trenching, are operations where the cemented carbide tipped tools at one moment are in engagement with the rock or ground and in the next second rotating in the air, often cooled by water. This causes a lot of thermal fatigue stresses as well as mechanical stresses, leading to microchipping and fracturing of the cemented carbide surface, often in combination with rapid high temperature abrasive sliding wear of the tip.
Pressure increases from 0 to 10 tons and temperature increases from room temperature up to 800.degree. C. or 1000.degree. C. in 1/10th of a second are generated at the contact zone between rock and cemented carbide tool tip when the tool enters the rock. This is not unusual today when stronger machines are used at higher cutting speeds in combination with harder and harder minerals, coal or ground to cut. Also, in those percussive or rotary rock drilling applications where extreme heat is being generated, like when drilling in iron ore (magnetite), rapid formation of thermal cracks, so-called "snake skin", occurs.
The properties which are absolutely essential to improve and optimize in the cutting material, i.e., the cemented carbide are:
thermal conductivity--the materials' ability to lead away or conduct heat, which must be as high as possible; PA1 thermal expansion coefficient--the linear expansion of the material when heating should be low to assure minimum thermal crack growth rate; PA1 hardness at elevated temperatures must be high to ensure a good wear resistance at high temperatures; PA1 transverse rupture strength (TRS) must be high; and PA1 fracture toughness--the ability of a material to resist catastrophic fracturing from small cracks present in the structure must be high.
It is well-known that the binder metal in cemented carbide, i.e., cobalt (nickel, iron) has a low thermal conductivity and a high thermal expansion coefficient. Therefore, the cobalt content should be kept low. On the other hand, a cemented carbide with high cobalt has a better strength, TRS and fracture toughness, which also is necessary from a mechanical point of view especially when high impact and peak loads are brought to the cemented carbide tip when entering the rock surface at high speed or from machine vibrations under hard cutting conditions.
Also known is that a coarser grain size of the WC phase is beneficial to the performance of the cemented carbide under conditions mentioned above, because of the increased fracture toughness and transverse rupture strength in comparison with more fine grained cemented carbides.
A trend in making tools for mining applications has therefore been to both lower the cobalt content together with increasing the grain size, thus achieving both a fair mechanical strength as well as acceptable high temperature wear properties. A larger grain size than 8-10 .mu.m at a Co content down to 6-8% is not possible to make with conventional methods because of the difficulty to make coarse WC crystals and because of the milling time in the ball mills needed for the necessary mixing of Co and WC and to avoid harmful porosity. Such milling leads to a rapid reduction of the WC grain size and a very uneven grain size distribution after sintering. During sintering, small grains dissolve and precipitate on already large grains at the high temperatures needed to achieve the overall grain size. Grain sizes between 1-50 .mu.m can often be found. Sintering temperatures from 1450.degree.-1550.degree. C. are often used, which also are needed to minimize the risk for excessive porosity because of the low Co contents. An unacceptably high porosity level will inevitably be the result of a too short milling time and/or lowering the cobalt content under 8 weight %. The wide grain size distribution for the coarse grained, conventionally produced cemented carbides is in fact, detrimental for the performance of the cemented carbide. Clusters of small grains of about 1-3 .mu.m as well as single abnormally large grains of 30-60 .mu.m act as brittle starting points for cracks like thermal fatigue cracks or spalling from mechanical overloading.
Cemented carbide is made by powder metallurgical methods comprising wet milling a powder mixture containing powders forming the hard constituents and binder phase, drying the milled mixture to a powder with good flow properties, pressing the dried powder to bodies of desired shape and finally sintering.
The intensive milling operation is performed in mills of different sizes using cemented carbide milling bodies. Milling is considered necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure during sintering. The milling time is in the order of several hours up to days.
The microstructure after sintering in a material manufactured from a milled powder is characterized by sharp, angular WC grains with a rather wide WC grain size distribution often with relatively large grains, which is a result of dissolution of fine grains, recrystallization and grain growth during the sintering cycle.
The grain size mentioned herein is always the Jeffries grain size of the WC measured on a photograph of a cross-section of the sintered cemented carbide body.
In U.S. Pat. Nos. 5,505,902 and 5,529,804, methods of making cemented carbide are disclosed according to which the milling is essentially excluded. Instead, in order to obtain a uniform distribution of the binder phase in the powder mixture, the hard constituent grains are precoated with the binder phase, the mixture is further mixed with a pressing agent, pressed and sintered. In the first mentioned patent, the coating is made by a SOL-GEL method and in the second, a polyol is used. When using these methods, it is possible to maintain the same grain size and shape as before sintering, due to the absence of grain growth during sintering.